How to Determine the Number of Lighting Branch Circuits?

Electrical Technology

How to Determine the Number of Lighting Branch Circuits for Dwelling and Non-Dwelling Units – NEC 2023

According to NEC Article 100 – Definitions, a branch circuit refers to the conductors between the overcurrent protection device and the final circuit element, such as an outlet. A general-purpose branch circuit supplies more than one outlet or receptacle for general lighting purposes. Furthermore, an individual branch circuit is defined as a branch circuit that supplies power to only one specific piece of utilization equipment.

In the following step by step guide, we will show how to know the number of general-purpose lighting branch circuits in a dwelling and non-dwelling building with solved example based on NEC-2023.

Good to Know: While determining the number of branch circuits, refer to 220.14 for other load, 220.16 for existing or additional installation, 220.41 for dwelling units, 220.42 for lighting loads for non-dwelling units and 220.44 for hotel and motels.

Related Posts: How to Determine the Number of Branch Circuits? – 3 Ways

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Lighting Branch Circuits in a Dwelling Unit

To determine the number of lighting branch circuits required in the dwelling and non-dwelling units, the National Electrical Code (NEC) 2023 provides specific guidelines in Article 220 and associated tables for branch circuits and lighting load for specified occupancies.

Article 220.41 specifies that the minimum load requirement for a dwelling unit is 3 VA (Volt-Amperes) per square foot (3 VA/ft²), which shall not be less than 33 VA/m² of occupied area.

Based on these values, the minimum requirements are as follows this equals to:

One 15-Amp lighting branch circuit for every 600 ft².
One 20-Amp lighting branch circuit for every 800 ft².

Since 20-Amp branch circuits are uncommon for lighting in residential units, we typically use 15-Amp lighting branch circuits.

Example 1

The following formulas can be used to determine the minimum number of 15-Amp lighting branch circuits required for a residence:

Suppose the total occupied area of a single-family unit with a basement is 3,000 ft² (278.7 m²). To find the total amperes required:

Amperes = (3VA × ft²) / 120V

(3VA × 3000 ft²) / 120V = 75-Amp

Now, Divide the calculated amperes by the rated amperes of the branch circuit (15 Amps).

Min. number of 15A lighting branch circuits = Amperes ÷ 15A

= 75A ÷ 15A

= 5

Alternatively, you may divide the total area in ft² by 600 ft² to get the number of 15A lightning branch circuits, and hence, will get the same answer i.e. 5.

Alternatively, you can divide the total area in ft² by 600 ft² to get the number of 15-Amp lighting branch circuits. The result will be same as above

3000 ft² ÷ 600 ft² = 5

Thus, the total number of required lighting branch circuits for a dwelling unit with a total occupied area of 3,000 ft² is 5.

Related Post: How to Find the Number of Outlets on a Single Circuit Breaker?

Lighting Branch Circuits For Non-dwelling Unit

The general lighting load for non-dwelling units (where the VA/ft² or VA/m² differs from that of a dwelling unit) with floor area determined by 220.5(C) can be calculated using Table 220.42(A). If the total load exceeds 3 kVA, a demand factor specified in Table 220.45 applies to the total branch circuit load for illumination. However, this is not applicable when determining the number of branch circuits for general lighting.

Example 1: Office Space

Given:

- Area = 3,000 ft²
- Lighting load = 1.3 VA/ft² (from NEC 2023 Table 220.41(A) for office buildings)
- Voltage = 120V
- Circuit Breaker = 15A

Steps:

1. Total Lighting Load:

2. Circuit Capacity:

Maximum load per circuit:

3. Number of Branch Circuits:

Divide total load by capacity:

Which is round up to 3 circuits.

Related Post: How to Find the Number of Lights on a Single Circuit Breaker?

Example 2: Retail Store

Given:

- Area = 2,500 ft²
- Lighting load = 1.9 VA/ft² (from Table 220.41(A) for retail spaces)
- Voltage = 277V (common in commercial buildings)
- Circuit Breaker = 20A

Steps:

1. Total Lighting Load:

2. Circuit Capacity:

Maximum load per circuit:

3. Number of Circuits:

Divide total load by circuit’s capacity:

Related Post: How to Determine the Number of Circuit Breakers in a Panel Board?

Good to Know:

NEC 210.11(A): The minimum number of branch circuits shall be determined by the size or rating of the circuit based on the total calculated load.
NEC 210.11(B): Loads calculated based on VA/ft² or VA/m² shall be evenly divided among multi-outlet branch circuits supplied by the panelboard(s).
The load calculation includes all general-purpose 20-Amp outlets and receptacles as specified in NEC 210.11(B), 210.52(E) & (G), and 210.70.
Motors rated below 0.5 HP are considered part of the general lighting circuit.
In all installations, the number of circuits shall be sufficient to supply the load served. In no case shall the load on any circuit exceed the maximum determined by Article 220.11.
NEC 240.4(D) specifies the maximum overcurrent protection ratings for conductors:
- 15-Amp for 14 AWG Copper or 12 AWG Aluminum
- 20-Amp for 12 AWG Copper or 10 AWG Aluminum
- 30-Amp for 10 AWG Copper